There has long been a need for relatively inexpensive but very strong permanent magnets. Therefore, considerable work has been done on the development of alloys and processes for making magnets of exceptional strength.
Before this invention, sintered or bonded samarium-cobalt (Sm-Co) powder magnets have been used in applications where high magnetic remanence and coercivity are needed in a shaped permanent magnet. However, such Sm-Co powder magnets are very expensive. The high price is a function of both the cost of the metals and the cost of their manufacture into magnets. Samarium is one of the least abundant rare earth elements, while cobalt is a critical metal with unreliable worldwide availability.
Processing Sm-Co powder magnets involves many critical steps. One such step is grinding alloy ingot into very fine powder. Ideally, each powder particle is a single crystal that is inherently magnetically anisotropic. To obtain an oriented permanent magnet, the anisotropic powder particles must be oriented in a magnetic field before the position of each particle is fixed by sintering or bonding. After sintering or bonding, the magnet must be finally magnetically aligned in the same direction in which the particles were initially oriented to obtain optimum magnetic properties, That is, the magnets are anisotropic. Sintered Sm-Co magnets may approach densities nearing 100% of alloy density. For bonded Sm-Co magnets, however, it is difficult to obtain densities much greater than about 75%. Conventional powder metal compaction equipment is not capable of achieving higher packing densities because of the shape and hardness of the powder particles.
This invention relates to high density, bonded, rare earth-transition metal magnets with properties nearly rivaling bonded samarium cobalt magnets. However, these novel magnets are based on the relatively common and inexpensive light rare earth elements, neodymium and praseodymium; the transition metal element, iron; and boron. These alloys and the method by which they are processed to achieve superior hard magnetic properties are described in detail in U.S. Ser. No. 414,936 now U.S. Pat. No. 4,851,058 by John Croat, a co-inventor of this invention. The application is assigned to the assignee hereof.
For use in this invention, the magnetic alloys are made by melt-spinning. Melt-spinning is a process by which a molten stream of alloy is impinged on the perimeter of a rotating quench wheel to produce rapidly quenched alloy ribbons. These ribbons are relatively brittle and have a very finely crystalline microstructure. They may be compacted and bonded as will be described hereafter to create novel, isotropic, high density, high performance permanent magnets.